In U.S. Pat. No. 7,259,367 B2, which is incorporated by reference, rain sensing by means of a camera is proposed, said rain sensing providing extensive lighting of the passing-through window of the camera aperture angle with the pane by means of an infrared diode. The camera focus is set to almost infinite and can thus be simultaneously used for driver assistance applications. Due to the imaging on the remote range raindrops are only noticeable as disturbances in the image, which are detected by complex differential measurements of the images recorded with infrared light pulsed or modulated in synchronization with the pixel clock.
A device and a method for detecting rain are described in WO 2012/092911 A1, which is incorporated by reference herein. A camera is disposed behind a pane, in particular in the interior of a vehicle behind a windshield, and focused onto a remote region that lies in front of the pane. A lighting source for generating at least one light beam that is directed at the pane directs the at least one light beam towards the window such that at least one beam that is reflected from the outer face of the pane impinges on the camera as an external light reflex or external reflex. The light quantity of the at least one beam or light reflex that impinges on the camera can be measured by the camera. The lighting source can generate light in the visible wavelength range. It must be ensured that road users are not disturbed by the lighting. To this end, the intensity and duration of the lighting can be adjusted to the external brightness.
However, lighting with visible light, even if it complies with the lighting guidelines for vehicles, can be deemed to be annoying. Other road users, in particular pedestrians, could be disturbed by such lighting, if they notice it.